Hardiness zone question

thomis(7)March 24, 2008

I have been confused on this hardiness zone thing.. I live in an area where I can't tell for sure if I am in zone 7 or 8 because I appear to be near the line. I wish I could find a zone map that had cities and towns overlain on it.

Anyways, what is the most important thing about the zone? Is it the cold temp or the warm temp that is important? For instance, a cherry tree I bought is listed for zones 5-7. Is that the hardiness rating for cold-hardiness or how much heat it can withstand?

Lastly, is there such a internet site or tool that one can use to determine how many chill hours are in your area?

There is more than one type of zone map. I believe it is the USDA zone map that has zones 1 to 10. These are now divided into A and B. The USDA zones are about winterhardiness. They are areas with equal average winter low temperature. It's really not all that accurate so it doesn't matter if you are 7B or 8A. Lately winters have been mild so the zones may be shiffting some.

Chill hrs are of more concern. But many new varieties don't have an accurate number. Also some fruits like apples are much more adaptable to low chill than previously thought. So to some extent it is a matter of trial and error.

The word "zone" (by itself) almost always refers to the USDA Plant Hardiness Zone. This rating is primarily useful to describe how cold tolerant a plant is. There are 11 zones (not 10) in this system. The zone rating refers to the average annual minimum temperature. When a plant is given a range (like zone 7b-10), it means the plant usually won't survive where temperatures drop below the lowest temperatures usually experienced in zone 7b AND probably wouldn't do well where temperatures were higher than those experienced in zone 10. Obviously there are shortcomings with this system. Temperature is definitely not the only important factor in how much heat or cold a plant can take and the average minimum temperature rating does not address how hot an area gets (and stays) during the warmer months. I have provided a link to the USDA Plant Hardiness Zone Map below.

In an attempt to overcome some of the disadvantages of depending on the upper number in the cold hardiness rating system for plants being grown in different climates, the American Horticultural Society came up with a Heat Zone rating system. It indicates the number of days per year above 86 degrees F (30 degrees C). Note that this zone rating system should always be referred to as Heat Zone. A map can be found at http://www.ahs.org/pdfs/05_heat_map.pdf

With the climate change that has occurred over the last couple of decades, weather patterns have shifted quite a bit. Many areas are warmer than they were when the USDA's map was produced. Some areas have actually even gotten COLDER! The National Arborday Foundation has attempted to "update" the Plant Hardiness Zone Map. Many people point out that there are various problems with this new map, but it is used by some people. It can be found at http://www.arborday.org/media/zones.cfm

The chill hrs map at Raintree is interesting. It even shows the decrease in chill hrs in North Dakota caused by the fact that they stay frozen all winter. The map is probably a pretty good estimate for areas east of the Rockies. But it is hopeless in areas with large variations in elevation like the Rockies. It is also hopelessly usless in California with all their microclimates. There are other zone maps for California. One is I believe the Sunset zones. In California you get into the tropical and subtropical fruits. That adds another layer of complexity. But all interesting stuff!!

When I was growing up, we were in a solid zone 4. When buying plants, if it was rated for a minimum zone 3, we figured the odds were 50/50 it might survive for us. If it was rated for a minimum zone 5, we figured it was highly experimental and was probably a waste of money, but might be fun to try.

We had other things going against us than just minimum winter temperature--moderate elevation, 4,500 feet, and low humidity, which makes for large temperature swings from day to night throughout the year. Thus, 70 degrees in early May might bring well below freezing at night, and anything in the 60's was a good chance of frost at night.

The point of all this, unless you are in an otherwise really odd climate, plants rated for a minimum of zone 7 will probably be hardy, those rated for zone 8 will probably do well but might have problems some years. Those rated for zone 6 or below and going up 8 or higher will probably be hardy. Some plants just won't like your climate no matter what, so you have to experiment with kinds and varieties to find the ones that like it best. Check your local extension service office for a list of plants recommended for your area.

Very nice. I have said it before and I'll say it again. I sure do wish I had found this forum back when I was planning my orchard last year.

The Arbor day map is the one that I appear to be right on the 7/8 zone line. But like you all said, there are other factors. I am particularly suprised at the chill hour map. I thought I was in an area with around 600 chill hours which heavily influenced which apple varieties I planted. That map says I have more than 1200! I'll just have to see. Only time will tell.

Sunset also puts out a national gardening book with zones assigned for the east. The amount of detail given regarding some of the western zones, expecially in California (for example, noting effects of frequent winds on plants in some zones) is remarkable. I don't know if this much detail was lavished on assignment of zones in the east.